1. Field of the Invention
The present invention relates to an internal combustion engine having two superchargers.
2. Description of the Related Art
To enhance the charging ratio of air, there is an engine having a turbocharger. The turbocharger includes a turbine and a compressor. The turbine is provided in a discharge passage. The compressor is provided in an intake passage. The compressor is driven in association with the turbine, and supercharges intake air into a combustion chamber.
In an engine of this kind, the turbine is rotated by exhaust gas flowing through the discharge passage. The compressor is driven in association with the turbine, and the intake air is supercharged into the combustion chamber. With this, the charging ratio of air is enhanced.
However, when the number of revolutions of the engine is in a predetermined rotation range, for example in a high rotation range, the turbocharger is set such that the air is supercharged. Thus, when the number of revolutions of the engine is out of the effective rotation range, no effect can be obtained.
To solve the inconvenience of the turbocharger and to enhance the charging ratio even when the number of revolutions of the engine is out of the effective rotation range, there is proposed an engine having a turbocharger and an electric supercharger. Jpn. Pat. Appln. KOKAI Publication No. 2004-76659 discloses such an engine.
The electric supercharger includes a compressor and an electric motor. The compressor is provided in an intake passage. The electric motor drives the compressor. Thus, the electric supercharger can supercharge the intake air into the combustion chamber irrespective of the number of revolutions of the engine.
According to the engine of this kind, when the number of revolutions of the engine is low, the charging ratio of air is enhanced by the electric supercharger. In the engine of this kind, when the number of revolutions of the engine is high, the charging ratio of air is enhanced by the turbocharger.
To clean the exhaust gas, there is an engine having a catalyst in the discharge passage. When the catalyst is heated to active temperature by the exhaust gas, the catalyst cleans the exhaust gas.
However, at the time of start of the engine, the catalyst is not heated yet. Thus, the exhaust gas is not cleaned and discharged out until the temperature of the catalyst reaches the active temperature.
Especially like the engine disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2004-76659, if the turbine of the turbocharger is provided in the discharge passage, the exhaust gas must also heat the turbine. That is, the heat capacity of the exhaust system is increased. Thus, the time elapsed until the temperature of the catalyst reaches the active temperature is increased. Therefore, it is conceived that most of the exhaust gas is not cleaned and discharged out.
Thus, there is proposed a secondary air supply system for heating the exhaust system. The secondary air supply system supplies secondary air to the discharge passage. Unburned fuel in the exhaust gas is burned in the discharge passage by the secondary air supplied to the discharge passage. As a result, the exhaust system is heated.
The secondary air supply system of this kind is separately provided with a compressor for supplying compressed air into the discharge passage. However, if the compressor is separately prepared, the cost is increased.
Hence, it is an object of the present invention to provide an internal combustion engine having a supercharger capable of effectively cleaning exhaust gas while restraining the cost.